It is known to use construction lasers (also called rotation lasers) at construction sites e.g. of buildings, in order to define reference points and reference lines on surfaces such as e.g. walls, floors or ceilings in particular in the case of work for interior finishing such as installations of pipelines and power lines or windows, positioning furniture, hanging pictures, etc. In particular, rotation lasers are used in which a laser beam (in the visible or infrared wavelength range) emitted by a laser unit produces a reference surface by deflection via a rotating deflection prism, a precise height reference then being provided by said reference surface. If a laser beam in the visible spectrum is emitted in this case and if this rotating laser beam in this case impinges on a surface such as e.g. a wall, a floor or a ceiling of the building, a reference line is visible there as a basis for further measures.
For precisely transmitting the reference height specified by the laser surface e.g. onto a wall, laser receivers capable of being handheld are known which can determine and indicate a position relative to a reference surface highly accurately.
Laser receivers capable of being handheld that are known from the prior art and serve for determining a position relative to the reference surface can in this case have a laser beam detector comprising a multiplicity of photosensitive elements, said laser beam detector being designed for generating an output signal when the laser beam impinges on the laser beam detector. In detail, the laser beam detector is designed in this case such that in addition an impingement position of the laser beam on the laser beam detector surface can be derived, for which purpose the photosensitive elements—considered in an upright operating position of the apparatus—can be strung together in a vertically aligned sensor line. In addition, usually an evaluation unit for determining the position of the laser receiver relative to the reference height defined by the rotating laser beam on the basis of the output of the laser beam detector and also an indicator for the position determined (for instance a visual display), in particular designed for indicating whether the laser receiver exactly coincides with the reference surface, are integrated into the laser receiver apparatus. In this case, the position can be determined for example on the basis of a ratio of a plurality of output signals (e.g. as the midpoint of that region on the laser beam detector surface which is illuminated by the laser beam).
Such laser receivers capable of being handheld can be used, in particular, when the line imaged on the wall by the rotating laser beam is discernible only with difficulty or not precisely enough by eye. This is e.g. the case for instance when there are relatively large distances between rotation laser and the wall (e.g. owing to a divergence of the laser beam [→imaged line becomes too wide] or a low light power [→imaged line becomes too weakly visible], which are set in this way in particular for eye safety reasons) or when laser light in the non-visible wavelength range is used.
In such cases, laser receivers of this type then make it possible to find the laser beam and to indicate and read the laser plane (or reference height) defined by a rotating laser beam and to transmit the height information onto a wall. By way of example it is possible—in a manner indicated by the laser receiver—to apply a corresponding marking (such as a pencil line) at the reference height on the wall.
For this purpose, on the part of the user, the laser receiver is for example moved up and down in a searching manner in a vertical direction and, finally, is brought into that position in which the indicator indicates coincidence with the reference surface. By way of example, the indicator provided can be a visual display which (for instance by means of luminous arrows or different-colored LEDs) gives information about whether a defined zero point of the laser receiver (e.g. a surface midpoint of the detector surface) is situated                exactly at the level of the reference surface,        above the reference surface or        below the reference surface.        
Examples of such laser receivers are disclosed in the documents EP 2 199 739 A1 and U.S. Pat. No. 4,240,208.
In order to provide for the user a simple transmission of the reference height determined and indicated by the laser receiver, a height mark can be provided on the housing of the laser receiver at the level of the defined zero point (e.g. a notch or an imprinted line laterally on the housing).
As described in EP 2 199 739 A1, the laser receiver can also itself have a laser fan emitter in such a way that a horizontal laser fan is emitted laterally at the level of the laser plane which is defined by the rotating laser beam and which is detected on the part of the receiver. Consequently, the height information defined by the rotating laser beam is amplified and/or forwarded. This makes it possible to generate sharp and clear reference lines e.g. on a wall, even though the rotation laser itself is far away or even situated in another room.
U.S. Pat. No. 7,394,527 discloses a system comprising laser emitter and laser receiver, wherein a distance between the laser receiver and the laser emitter is intended to be determined. For this purpose, it is proposed to emit two mutually parallel laser beams in a rotating fashion and to determine the distance depending on rotational speed and time offset of the directly successively received laser pulses of the two laser beams. Analogously to this—given the presence of a plurality of detector strips offset parallel to one another on the part of the receiver (with a precisely known parallel offset of the detector strips with respect to one another)—alternatively it is also possible to emit a single laser beam in a rotating fashion, in which case the distance between the receiver and the laser emitter is then determined depending on the time offset of the laser pulses received successively by the respective detector strips.
U.S. Pat. No. 5,953,108 discloses a system comprising rotation laser and laser receiver, wherein the laser beam rotates at a first speed if no message is being communicated to the laser receiver, and rotates at a second speed, different from the first rotational speed, in order thereby to communicate a predefined message with regard to a status state of the rotation laser (e.g. “weak battery”).
In all of the systems described previously, however, it can happen that when more than one rotation laser is used on a construction site, the laser receiver receives and identifies a laser beam emitted by a different rotation laser from the one actually intended. Therefore, there is the risk that an incorrect reference height will be indicated by the laser receiver, this incorrect reference height will be transmitted onto a wall and as a result, finally, possibly even the entire construction project (such as e.g. installation of windows at a predetermined room height) will be performed erroneously.
Laser receivers are additionally known which are precisely designed for and coordinated with rotation lasers having certain properties and react and ultimately indicate the height of a laser plane only if the rotation laser generating the laser plane meets certain requirements, e.g. if the fixed rotational speed of the rotation laser lies within a predetermined range, if the wavelength of the laser beam lies within a certain wavelength range or if the received laser beam is modulated with a defined modulation scheme. A use of an apparatus combination comprising rotation laser and laser receiver which, however, are not already designed for and coordinated with one another in respect of hardware is thereby made impossible, however, even if this ensures, in principle, a relatively simple and unambiguous assignment of the apparatuses on a construction site. By way of example, U.S. Pat. No. 6,052,181 discloses a system comprising rotation laser and laser receiver in which the laser beam emitted by the rotation laser is amplitude-modulated and contains a modulated identification data stream and other user data. The laser receiver demodulates the received laser beam immediately after the latter impinges thereon, and has a special detector for reading out the modulated data and thus for identifying or indicating the height level.